1. Field of the Invention
The present invention relates to pressure sensors or more particularly to pressure sensors for use in wet environments.
2. Description of the Related Art
In wet applications, a pressure sensor comes into contact with fluids and gases having high moisture content. This moisture damages the wire bonds that are used for bonding the sensor and piezoresistors that are doped on the surface of the sensor. As a result, silicon piezoresistive sensing technology has heretofore only been used directly for dry media applications and has been used by protecting it with epoxy for wet applications. However, there is also a great need in the art for measuring pressures in wet media directly as epoxy interferes with the measurements. There is also a need for an amplified pressure sensor for such applications. Some applications additionally need to have signal conditioning/signal amplification incorporated in sensor packages. Most wet differential pressure sensors employ a metal capacitive pressure sensor. Currently there is no wet differential amplified pressure sensor based on piezoresistive sensing technology with a surface mounted technology (SMT) package. A SMT packaged pressure sensor provides a small, low cost, high value pressure sensor that could be mounted on a printed circuit board (PCB) with smaller pad dimensions, thus ensuring less space occupied on the PCB. This also helps in lowering installation costs and eliminates secondary operations. This sensor could meet the needs of many medical sensor applications that need a SMT package and also where an amplified pressure reading is used.
It is known in the art to produce sensor packages. U.S. Pat. No. 6,117,292 provides a chemical sensor apparatus in which an ion sensitive silicon die is sandwiched between an elastomeric media seal and a conductive elastomeric pad contained within a housing. U.S. Pat. No. 6,153,070 provides a sensor packaging using a heat staking technique. An environmental sensor apparatus is disclosed in which an environmentally sensitive silicon die is sandwiched between an elastomeric media seal and a conductive elastomeric pad contained within a housing. U.S. Pat. No. 5,184,107 provides a piezoresistive pressure transducer with a conductive elastomeric seal. A piezoresistive pressure transducer uses pre-molded elastomeric seals in which at least one seal is electrically conductive. A piezoresistive stress sensitive element in the form of a diaphragm of semiconductor material having a thickened rim is held at its rim between a pair of pre-molded elastomeric seals in a two-piece housing. None of these employ a pressure sensor positioned in one compartment of a housing and a signal amplifier positioned in a separated compartment of the housing so that the signal amplifier is separated from a relatively harsh, wet, pressure sensing environment.
It has now been found that an improved sensor package can be prepared by partitioning a plastic housing into two halves. In one of the halves, a pressure sensor is sandwiched between an electrically conductive seal and elastomeric media seal and in the other half a signal amplifier in the form of an application specific integrated circuit (ASIC) is provided. The cap over the ASIC is ultrasonically welded while the cap over the pressure sensor can be ultrasonically welded or heat staked. The conductive seal can replace wire bonds for establishing electrical contact between the pressure sensor and the lead frame. The elastomeric seal, i.e., media seal is used to isolate the fluid from attacking the metal contacts on the sensor surface. To incorporate amplification to the sensor output, an ASIC/instrumentation amplifier/operational amplifier can be employed. An ASIC may bring about signal conditioning along with amplification. When an ASIC is incorporated in this type of arrangement it would avoid contact with the fluids or moisture present in the gases which could damage the ASIC functionality. In this regard, the mechanical packaging arrangement of the invention partitions the housing into two halves. In one of the half the pressure sensor with the conductive seal and media seal is packaged and in the other half an ASIC is packaged.